Temperature compensation for ground piercing metal detector

ABSTRACT

A hand tool has a ground-piercing probe, which is manually insertable in the soil. The probe has a chamber in which an inductor is positioned and connected to metal detection circuitry. A pulse generator applyies current pulses to an inductor for inducing eddy currents in a buried target object. After the pulse terminates, the decaying coil voltage is sampled at different times to detect both the presence and range of the object, as well as the type of metal in the object. The coil voltage samples are applied to signaling circuitry, which provides an audible signal which is a series of bursts of an audio tone. The pitch of the audio tone signals the presence, range and bearing of the buried metal object, while the pulse rate of the bursts signals the metal type. Signal variations resulting from temperature changes when the probe is inserted into soil are compensated for by detecting a signal proportional to the coil current during the pulse and subtracting a scaled portion of that signal from voltage samples.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application is a continuation in part of copendingapplication Ser. No. 09/366,805 filed Aug. 4, 1999 and entitled GroundPiercing Metal Detector Having Range, Bearing and Metal-TypeDiscrimination.

STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT

[0002] (Not Applicable)

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] This invention relates generally to metal detectors for detectingburied metal objects and more particularly relates to a metal detectorusing a ground piercing probe and associated electronic circuitry andmethods for detecting not only the range of buried metal objects, butalso the bearing and metal type of the buried metal object.

[0005] 2. Description of the Related Art

[0006] Metal detectors have long been used by hobbyists as a favorableform of recreation, which offers not only an enjoyable activity, butalso the opportunity to discover valuable and/or historical metal targetobjects buried in the soil.

[0007] In conventional prior art metal detecting, a user laterallyreciprocates a metal detector, usually including a coil or otherinductor, above a soil surface in a scanning pattern seeking an audiblesignal which indicates the presence, under the soil surface, of a metalobject. The soil in which such buried metal objects are sought includesnot only dirt and sand, and other manually movable or deformable earthsurface materials, which are readily accessible to humans, but alsoincludes underwater lake and sea bottoms.

[0008] A commonly used signal which indicates the presence of a metalobject is an increase in the frequency of an audible tone. The typicaluser reciprocates the inductor of the typical metal detector above thesurface in repetitive left and right, arcuate reciprocating movementsuntil the frequency increase is heard or a tone is heard. Upon hearing afrequency increase, the user then begins reciprocating the inductor insmaller arcs across the region where the increased frequency signal wasdetected in an attempt to more precisely determine a position verticallyabove the buried metal object.

[0009] When permitted, a shovel is then used to dig up a clump of soiland the user then breaks apart and sifts through the clump using theuser's fingers or a tool in an attempt to find the metal target object.If the user is fortunate, the target object will be found in the clumpof soil which has been dug up. Unfortunately, conventional metaldetectors often cannot sufficiently accurately pinpoint the location ofthe buried metal object. Therefore, the metal detector is again employedand additional clumps of soil must be similarly dug up, sifted throughand inspected. Also, the buried metal object may be located below thedepth of the initial dig, necessitating deeper digging to retrieve theobjects. Consequently, conventional metal detecting typically requiresextensive manual labor for removing clumps of soil, breaking them apartand sifting through them.

[0010] After completing these activities, a conscientious metal detectoruser will then carefully replace the soil in an attempt to return thesoil as nearly as possible to its original condition. However, someusers do not exercise such care, merely leaving a hole in the soil. Evenusers who carefully replace the soil have nonetheless created and leftbehind a substantial environmental disturbance in the soil.

[0011] These soil disturbances, especially at popular historical sitesand highly trafficked outdoor public or park areas, often cumulativelycause both significant damage to the visual, cosmetic appearance of suchareas as well as destruction of vegetation or other components of thelocal ecosystem and the creation of safety hazards.

[0012] As a result, many owners of private land and operators of publicparks have imposed digging restrictions and regulations to minimize suchdamage or destruction. Typically, these restrictions limit the metaldetector operator to digging in the soil with a tool no larger than ascrewdriver and to digging only small holes to retrieve a metal targetobject. Because conventional metal detectors are insufficiently accurateto allow the target metal object to be located within a distance rangetolerance of such a small hole, the operators use a probe, such as anice pick or a screwdriver, to pierce the ground in the vicinity of thedetected object in an attempt to strike the object and feel itspresence. Such ground piercing does not cause significant soildisturbance and often is beneficial in providing aeration for trees orother vegetation. However, one problem with this technique is that it isa trial and error process which usually requires numerous groundpiercings because such a probe only has a detection width equal to itsown very narrow width. An additional problem is that such a probe andpierce technique does not enable the user to distinguish between thedetected metal object and a nearby stone, tree root or other buriedobject which is harder than the surrounding soil. Consequently, smalldigs resulting in retrieval of only a stone or exposure or damage to atree root, often occur. Furthermore, if an initial small hole has beendug and the object is not found, the problem remains for the user todetermine in which direction from the hole the buried target objectlies.

[0013] In an attempt to overcome the inherent inaccuracy of theconventional metal detector, the prior art has provided miniaturizedmetal detectors. These are of the same type as and modeled after thelarger conventional metal detectors, but are much smaller. They aretypically larger than ½″ in diameter, have a plastic outer sleeve orshaft and use conventional metal detection circuitry. These miniatureversions of conventional metal detectors are used to search for buriedmetal objects near the walls of a hole the user has dug or in dirt theuser has dug out to form the hole and is sifting through.

[0014] There is therefore a need for a hand tool and associatedelectronic circuitry and methods to more precisely locate the buriedmetal target object after the general vicinity of its location has beendetected by a conventional metal detector.

[0015] An object and feature of the present invention is that, after thegeneral vicinity of a buried metal target object has been located by aconventional metal detector and before any holes have been dug,embodiments of the invention permit the location of the object to bemore precisely detected, utilizing considerably fewer soil piercingswithout significant disturbance of the soil.

[0016] Another object and feature of the invention is to provide a metaldetecting hand tool having a probe with sufficient strength, hardnessand toughness so that it can survive repetitive insertions intoundisturbed soil, especially hard soils and abrasive soils, such assand.

[0017] A further object and feature of the invention is that each timethe soil is pierced by the probe of the invention, the bearing of theobject may be detected to guide the user toward the next mostappropriate place to again pierce the ground, thus greatly improving theprobability that the metal object will be struck by the next groundpiercing.

[0018] A further object and feature of the invention is to detect notonly the range and bearing to the buried metal object, but to detectinformation about the type of metal in that object. This permits theuser to discontinue the effort to retrieve the metal object if the useris not interested in objects of the detected metal type and also permitsthe user to spend more time being more persistent if a potentiallyvaluable metal is detected.

BRIEF SUMMARY OF THE INVENTION

[0019] The invention is an improved hand tool for detecting buried metalobjects. The hand tool has a handgrip attached to a ground-piercingprobe, which is manually insertable in the soil to displace the soilradially outwardly from the probe. An improvement of the invention is achamber in the probe and at least one inductor positioned in thatchamber and connected to metal detection circuitry. The inventionincludes other features, such as the provision of an asymmetricallyshaped magnetic field pattern about the probe for providing directionalsensitivity and the use of particular materials in the probe.

[0020] The invention further is directed to a method for detecting thelocation of a buried metal target object. The method is initiated byplunging the probe of a tool embodying the invention a distance into thesoil to displace the soil outwardly from the probe and position thechamber below the surface of the soil. Eddy currents are then induced inthe target object by generating a time changing magnetic field about thecoil located in the chamber. Current induced in the coil by the eddycurrents is then detected to determine the presence and/or range of themetal object and, if certain additional features of the invention areutilized, the bearing and metal type of the target object.

[0021] The invention further is a metal detection circuit for detectingthe type of metal in the buried metal object. The circuit provides asignal to the user which indicates whether the metal is a ferrous metal,such as iron or steel, a high conductivity metal, such as pure gold, asilver coin or a copper coin, or a medium conductivity metal, such asaluminum alloys or gold alloys commonly used for jewelry. The metaldetection circuit has a pulse generator connected to a coil forgenerating a time changing magnetic field in response to electricalpulses applied to the coil. A resistive energy damper is coupled to thecoil for attenuating the energy in the magnetic field in a manner whichextends the decay of the current in the coil after termination of thepulse. A sampling and storing circuit is connected to the coil forsampling a signal representing the magnitude of the coil current at asampling time. Preferably coil voltage is sampled in order to detect thecoil current at the sampling time because voltage and current arerelated by ohms law and are therefore interchangeable signals.Consequently, most qualitative observations made about coil current arealso applicable to coil voltage. The sampling time for detecting metaltype is within a time interval beginning after termination of the pulsefrom the pulse generator and ending before the time at which the coilcurrent in the presence of a high conductivity metal target objectdecays to a current which exceeds the current to which the coil currentdecays in the absence of a metal target object. A signaling circuit isconnected to the output of the sampling and storing circuit forsignaling changes in the detected coil current as the coil is moved. Inparticular, the signaling circuit signals whether the coil currentincreases, decreases or remains substantially the same. The direction ofchange or the lack of change in the coil current as the coil is movedmay then be interpreted by the user, or a suitably programmed computer,as an indication of the type of metal in the buried metal object.

[0022] The invention further is a method for detecting the metal type ofa metal target object buried below the surface of a soil. The methodbegins by attempting to induce eddy currents in a target object byapplying a current pulse to a coil which is coupled to a damping load.The current induced in a coil by the eddy currents is detected at afirst sampling time. The first sampling time is located within a timeinterval beginning after termination of the current pulse applied to thecoil. The time interval ends before the time at which the coil current,in the presence of a high conductivity metal object, decays to a currentwhich exceeds the current to which the coil decays in the absence of ametal target object. Changes in the detected coil current are monitoredand signaled as the probe is moved. These signaled changes in the coilcurrent sample magnitude as the coil is moved closer to the metal objectare then interpreted by the user to determine the type of metal objectburied beneath the soil.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0023]FIG. 1 is a pictorial view illustrating the use of an embodimentof the invention.

[0024]FIG. 2 is a view in vertical section of an embodiment of theinvention.

[0025]FIG. 3 is a view in side elevation of an alternative embodiment ofthe invention.

[0026]FIG. 4 is an enlarged view in side elevation of an inductor formedby a coil and ferrite core which has been contoured to provide anasymmetrical magnetic field.

[0027]FIG. 5 is a view in horizontal section through a probe of analternative embodiment for creating an asymmetrical magnetic field.

[0028]FIG. 6 is a view in horizontal section through a probe of analternative embodiment for creating an asymmetrical magnetic field.

[0029]FIG. 7 is a block diagram illustrating the fundamental principlesof the present invention.

[0030]FIG. 8 is an oscillogram illustrating the principles of operationof the present invention.

[0031]FIG. 9 is a block diagram of the preferred circuit embodying thepresent invention.

[0032] FIGS. 10-13 are schematic diagrams of the preferred embodiment ofthe invention, which, because of the size and detail illustrated, mustbe broken among several sheets, but when connected together illustratethe preferred circuit of the present invention.

[0033]FIG. 14 is a view in axial section taken substantially along thelines 14-14 of FIG. 3.

[0034]FIG. 15 is a schematic diagram illustrating the preferredembodiment of the temperature compensation invention.

[0035]FIG. 16 is a detailed schematic diagram illustrating the detailedpreferred embodiment of the metal detector including the preferredtemperature compensation circuitry.

[0036]FIG. 17 is a schematic diagram illustrating a constant currentembodiment of the temperature compensation circuitry embodiment.

[0037] In describing the preferred embodiment of the invention which isillustrated in the drawings, specific terminology will be resorted tofor the sake of clarity. However, it is not intended that the inventionbe limited to the specific terms so selected and it is to be understoodthat each specific term includes all technical equivalents which operatein a similar manner to accomplish a similar purpose. For example, theword connected or terms similar thereto are often used. They are notlimited to direct connection but include connection through othercircuit elements where such connection is recognized as being equivalentby those skilled in the art. In addition, many circuits are illustratedwhich are of a type which perform well-known operations on electronicsignals. Those skilled in the art will recognize that there are many,and in the future may be additional, alternative circuits which arerecognized as equivalent because they provide the same operations on thesignals. Further, those skilled in the art will recognize that, underwell-known principles of Boolean logic, logic levels and logic functionsmay be inverted to obtain identical or equivalent results. Similarly,there are currently various materials existing, and are likely to beadditional materials developed in the future, which exhibit thecharacteristics described herein, making them useful in embodiments ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

[0038] Referring to FIGS. 1 and 2, the hand tool 10 of the presentinvention has a handgrip 12 attached to a ground-piercing probe 14. Theprobe 14 extends perpendicularly from a central portion of the handgrip12. The handgrip extends in opposite directions from an end of the probeto provide a t-shaped hand tool, so that the handgrip can be comfortablygripped by the hand of a user 16 and manually thrust into the soil 18.When inserted in the soil 18, the probe 14 causes the soil 18 to bedisplaced a small distance radially outwardly from the probe 14. The end19 of the probe 14 is preferably rounded and most preferablyhemispherical to facilitate its insertion into the soil and to providestrength and durability.

[0039] The probe 14 is preferably tubular, so that it is provided withan interior chamber 20. Alternatively, the chamber 20 may be formed as agroove, notch or cavity in the probe 14, but the tubular shape ispreferred because such a cavity within the tube provides both a chamberin which an inductor 22 may be mounted and through which a wire 24 mayeasily extend into connection with one terminal of the inductor 22. Aswill be described below, the probe 14 can be made of a variety ofmaterials and if the probe 14 is a metal, the other terminal of theinductor 22 may be connected directly to the metal probe 14, which is inturn connected to a wire 26. The wires 24 and 26 connect the inductor 22to a metal detection circuit 28. If the probe 14 is a nonconductivematerial, the wire 26 also extends into the chamber into connection withthe other terminal of the inductor 22. Preferably, the wires 24 and 26extend from the interior, central portion of the handgrip 12, outthrough the end 30 of the handgrip 12 and they are sufficiently longthat the metal detection circuit 28 can be conveniently, releasablymounted to the belt, or other clothing article, on the user 16 orself-contained and hand-carried.

[0040]FIG. 3 illustrates some alternative features of a hand toolembodying the present invention. FIG. 3 shows a hand tool having ahandgrip 32 and a probe 34 with length graduations 36 together withnumerals indicating length dimensions spaced along the probe 34, withreference to the rounded tip 38 of the probe 34. This allows a user todetermine the depth of an object which is struck by the probe. The usercan then choose the most effective tool for extracting the targetobject. The graduations also provide an indication of the depth of anobject which is not struck, but is detected by the electronic circuitryin a manner subsequently described as being located horizontally fromthe inserted probe.

[0041] As will be described below, the tool can also have a directionalsensitivity in a plane which is perpendicular to the axis of the probe14 or 34, as a result of providing a radially asymmetric field patternaround the inductor 22. Consequently, it is also desirable to provide adirection pointing indicium, such as an arrow 40 on the handle 12,indicating the direction of maximum sensitivity, which is the directionof the major lobe of the field pattern. This indicium, of course, canalternatively consist of a dot or other geometric figure placed at oneend of the handle, a different coloration or other indicium to indicatethe direction of maximum radial sensitivity.

[0042] Certain dimensional features and material characteristics of theinvention are particularly desirable and preferred. The diameter of theprobe 14 or 34 should be less than about ⅜″ because it is too difficultor impossible to manually force the probe into many soils if it is anylarger. Preferably, a tubular probe has an outside diameter ofsubstantially {fraction (7/32)}″ or 0.226″, which are equivalent. Thepreferred probe 34 has two component parts illustrated in FIG. 3 and inmore detail in FIG. 14. One component is a #304 stainless steel tube 35having an outside diameter of 0.226″, an inside diameter of 0.080″ and alength of 12.5″. The stainless tube 35 is telescopically interfit intoand bonded to a second tip component along an annular shoulder segment37 which is 0.50″ in axial length. The tip component is a tubularzirconia tip 39 which has a cylindrical portion 1.5″ long with anoutside diameter of {fraction (7/32)}″ and an inside diameter of 0.145″and a hemispherical tip to provide an overall probe length of 14″. Alonger zirconia tip is weaker and a shorter zirconia tip isinsufficiently long to both contain a coil and fit over the stainlesstube. Other materials would also make desirable tips. For example, thetip may be constructed of a metal oxide, such as sapphire, or a glass orceramic.

[0043] Most conventional metal detection circuits, as well as thepreferred metal detection circuit of the present invention, include aheadset connected by a wire to the main circuitry of the metal detectioncircuit to enable the user to hear an audio signal generated by themetal detection circuitry. Sometimes audio speakers are also providedfor alternative use. Alternatively, however, an FM transmitter 42 may beconnected in the principal metal detection circuitry 44 for modulatingthe audio signal generated by the metal detection circuitry 44 andtransmitting that signal to an FM receiver 46, mounted to the headset 48and electrically connected to its sound transducers. Of course, othertypes of modulation may be used.

[0044] As a further alternative, the metal detection circuitry may beminiaturized and mounted within the handgrip 12 or 32 of the presentinvention, including the FM transmitter, to entirely eliminate the needfor a wire extending out of the handgrip 12 or 32 to a remotely mountedmetal detection circuit.

[0045] Popular, prior art metal detection circuits utilize an inductor,usually in the form of a coil. The hand tool of the present inventionillustrated in FIGS. 1-3, can be used with these prior art metaldetection circuits with the appropriate selection of probe materials andthe coil. Preferably, however, the probe is used with the modified pulseinduction detection circuitry, illustrated in the Figures and describedbelow.

[0046] The prior art circuits, as well as the circuit of the presentinvention, detect the presence of a buried metal object by inducing eddycurrents in a buried metal object by means of a time varying current ina coil. The frequency of the coil current is sufficiently high toprovide acceptable coupling and a sufficiently low frequency to provideadequate soil penetration. The typical frequency of operation is 5 KHz.

[0047] One type of prior art metal detection system uses detuning of anoscillator as a result of inductive coupling between an inductor and aburied metal object. One embodiment of this system has a pair ofoscillators, referred to as beat frequency oscillators. One oscillatoris operated at a fixed frequency. The other oscillator is tuned tosubstantially the same frequency, but includes a coil, which is thesearch coil. In embodiments of the invention, the coil 22 in FIG. 2 maybe used as such a search coil. The outputs of these two oscillators aremixed to detect a difference audio frequency which provides an output,audio tone. As the search coil is brought into proximity with a buriedmetal object, the second oscillator is detuned in proportion to thesearch coil's coupling to the buried metal object. Consequently, changesin the pitch of the audio output tone signal the presence and range ofthe buried metal object. As the range decreases, the mutual inductivecoupling is increased causing the oscillator to be further detuned andtherefore raising the audio pitch.

[0048] Another variation of the oscillator detuning principle utilizes asingle oscillator connected to the coil, such as the inductor 22. Theoscillator has a conventional audio frequency control negative feedbackloop, having a signal which varies in magnitude in proportion todetuning of the oscillator by coupling of the coil to a buried metalobject. The magnitude variations of the AFC loop are used to control avoltage controlled oscillator, oscillating at an audio frequency, withthe result that the frequency of the audio output tone is varied as thecoil is brought into proximity with the buried metal object.

[0049] The coil 22 in embodiments of the invention may be used as thesensing coil with both of these circuits embodying the detuningprinciple, although this is not preferred.

[0050] A second type of prior art metal detection circuit uses theinduction balance principle. With such a circuit, two coils areorthogonally positioned to provide a null condition; that is so thatthere is no electromagnetic coupling between the coils. One coiltransmits to the buried object at an oscillator frequency, while theother is used for receiving a signal from the eddy currents induced inthe object. When no metal object is present and consequently there is nometal object in which eddy currents are generated, little or no signalis received by the receiving coil. However, the presence of a metalobject in proximity to the coils allows the eddy currents generated inthe metal object to be transmitted back to and received by the receivingcoil and detected by the metal detection circuitry. The magnitude ofthis received signal, coupled from the transmitting coil to the metalobject and then from the metal object back to the receiving coil, isapplied to a signaling circuit, typically an audio oscillator, to varythe audio tone in proportion to the magnitude of the received signal.This magnitude indicates the presence and range of the buried metalobject. The coils of an induction balance detection system may also bemounted in the probe 14 or 34 using the present invention, although thisis not preferred.

[0051] Both of the above described prior art metal detection systems,when they are utilized with the structure of the present invention,require that the probe be formed of a non-metallic or dielectricmaterial so that eddy currents are not generated in the probe itself.Suitable materials include a relatively hard, durable plastic material,a composite such as a carbon fiber/epoxy composite, or a ceramicmaterial, such as zirconia. The probe can also be constructed withmultiple sections of different material, such as metal tube having azirconia or other ceramic tip portion 39, illustrated in FIG. 3. Thisceramic tip construction is also particularly desirable in preferredembodiments of the invention.

[0052] The third type of metal detection circuitry known in the priorart uses pulse induction detection. That system ordinarily utilizes asingle coil for both transmitting and receiving although someembodiments utilize two coils, one for transmitting and a different onefor receiving. In the pulse induction detection circuitry, a pulse isapplied to a coil with a sharp cutoff to provide a step function which,as known to those skilled in the art, produces high frequencies. Thus,by pulsing the coil, a time changing magnetic field is generated aboutthe coil, which induces eddy currents in a metal target object. Theseeddy currents are then coupled back to either the same coil or a secondreceiving coil and the received signal is detected and applied to asignaling circuit. Since the magnitude of the received signal is afunction of the distance to a buried metal object, the signaling circuitsignals changes in received signal magnitude to indicate the presenceand range of the metal target. The present invention with the coil 22,illustrated in FIG. 2, may be utilized with prior art pulse inductiondetection circuitry. A second coil may be mounted in the probe 14 foruse with pulse induction circuits utilizing two coils.

[0053]FIG. 7 is a simplified block circuit diagram illustrating thefundamental principles of the present invention and FIG. 8 illustratesthe operation of that circuit. FIG. 7 uses the pulse induction systemand therefore has a pulse generator 50, which applies periodic pulses toinductor 52. The term “inductor” is used to indicate an electroniccomponent which exhibits an inductance. The term includes a coil with nocore material as well as a coil with a core. As is known in the art,separate transmitting and receiving inductors may be used, but are notpreferred because the circuit is more simplified by elimination of oneinductor.

[0054] The pulses applied to the inductor 52 may, for example, beapplied to the inductor 52 at a 120 Hz rate, with a pulse width of, forexample 150 microseconds, for driving the inductor 52 at a current of,for example, 1 amp. The choice of pulse rate results from an engineeringtradeoff between an increase in power consumption and coil heating asfrequency is increased and a decrease in sensitivity as the frequency isdecreased.

[0055] A resistive energy damper 54 is coupled to the inductor 52 eitherby direct conductive connection or by inductive coupling. The resistiveenergy damper 54 may be a resistor or a resistive network connected inshunt across the inductor 52. Alternatively, the energy damper may be anonferrous, high resistivity metal inductively coupled to the inductor52, such as a metal used to construct the probe of the presentinvention.

[0056] A sampling circuit 56 is connected to the inductor 52 forsampling the coil voltage or current at selected sampling timessubsequent to the termination of the pulse from the pulse generator 50.Signals representing the magnitude of the samples are then applied tosignaling circuits 58 and 60.

[0057] As illustrated in FIG. 8, the pulse terminates at time TO, afterwhich the voltage across the inductor 52 decays exponentially at a ratewhich is dependent upon the resistive energy damper, the presence orabsence of a buried target object, its range and the type of metal ofwhich the target object is constructed. FIG. 8 is a family ofoscillograms of exponentially decaying coil voltages for representativemetals and for the absence of a metal target. The metals representferrous metals, high conductivity metals and mid-range conductivitymetals. This family of oscillograms has the characteristic that there isan initial region of exponential decay having a duration of several timeconstants during which most of the decay of this family of curvesoccurs. This is followed by a region of slower decay rate.

[0058]FIG. 8 is a drawing of actual oscillograms. In the circuit fromwhich these oscillograms were derived, the voltage pulse is clamped by a15 volt zener diode to protect the input of an op-amp from a 200 voltspike. Consequently, the coil current, when the coil voltage exceeds 15volts, is shunted through the zener diode. Because of the clampingdiode, during the initial, relatively flat voltage segment of theoscillogram, the coil current is exponential but the voltage is not.This early region of shunted coil current is not used by the circuit asa signal.

[0059] One important characteristic of this family of curves is thatafter several time constants, the curve N, which represents the decay inthe absence of a metal target, decays to the lowest value of any of thedecaying curves. That is because there are no eddy currents induced in aburied metal object to store energy and couple energy back to the metaldetector circuit. In this region to the right of the knee of thesecurves, samples are taken to determine whether a metal object is presentand its range. For example, in the present invention samples are takenat time T4, which is preferably 34.1 microseconds after time T0, thetime of termination of the pulse applied to the inductor 52. The sampleduration is preferably 1 microsecond. Samples taken at this later timeinterval are applied to signaling circuit 60 for indicating targetpresence and range. As in prior art circuits, the magnitude of thesignals coupled from the eddy currents induced in the buried metalobject back to the receiving coil are an inverse function of the rangeto the metal object.

[0060] The reason that the inductor current takes longer to decay in thepresence of a metal object and therefore has a higher magnitude atsample time T4 is that energy is coupled to a metal object and generateseddy currents. These eddy currents generate magnetic fields in whichenergy is stored. Therefore, the pulse applied to the pulse generatorcauses more energy to be stored in the system in the presence of a metalobject, than in the absence of a metal object. Consequently, the decaytime is longer in the presence of a metal object.

[0061] At sampling time T4, although all decaying oscillograms whichrepresent metal objects are greater than the oscillogram N representingthe absence of a target metal object, there is only a small differencebetween them. Consequently, samples taken in this later region aredifficult to use for distinguishing between the type of metal detected.However, since the magnitude of samples, taken beyond the knee of thecurve at time T4, vary inversely to the range of a metal object, samplestaken at this later time are used to determine the presence and range ofa buried metal object.

[0062] The signaling circuit 60 therefore signals to the user changes orlack of changes in the magnitude of the samples taken, for example attime T4, as the inductor 52 is moved nearer or further from a buriedmetal object. These changes may be signaled, for example, by varying thetone of an audio oscillator in proportion to the magnitude of thosesamples, or by other signaling means, such as a numerical display, ameter, a light which varies in intensity, or other signaling circuitryor devices.

[0063] Another important characteristic of the decay oscillogramsillustrated in FIG. 8 is the subject of the present invention. The rateof decay of the voltage across the inductor 52 during the first few timeconstants of decay is also dependent upon the type of metal in theburied metal object. At times preceding the knee of the oscillograms,the decaying voltage for the different metals exhibit more substantialdifferences in magnitude than after the knee. Therefore, samples can betaken at a selected time in this earlier portion of the decay and usedto discriminate between different kinds of metals.

[0064] In order to provide a time interval which is sufficiently long toallow samples to be taken for detecting metal type, the time rate ofdecay must be reduced (i.e. the decay time extended) from that in priorart pulse induction systems. In the prior art pulse induction systems,the initial decay is so rapid that meaningful samples cannot be taken.

[0065] In the present invention, the decay rate is reduced (i.e. thedecay time and time constant are increased) by providing the resistiveenergy damper 54. Whether the resistive energy damper is a resistor,network of resistors or other resistance, or is a nonferrous, lowconductivity metal forming the probe, the effect of the resistive energydamper is to reduce the decay rate and thus extend the time interval inwhich samples may be taken to determine the type of metal. The resistiveenergy damper reduces the rate of decay because the time constant for aresistive/inductive circuit is L/R. Consequently, the time constant isinversely proportional to the resistance of the energy damper, if theenergy damper is a resistance connected across the inductor 52.

[0066] The use of a nonferrous metal probe, instead of a resistanceconnected across the inductor, has a similar effect in reducing the rateof decay and thereby extending the time interval before the knee of thecurves, during which samples can be taken for detecting metal type.However, the non-ferrous metal probe should be a relatively highresistance (low conductivity) metal to avoid extending the decay time ofthe eddy currents in the metal probe beyond the time it takes for eddycurrents in a metal target object to decay. The extension of the decaytime occurs because energy is stored in the magnetic field of the eddycurrents induced in the nonferrous metal probe material, prolonging thedecay time because more energy is stored in the coupled system. Thesystem can also be considered as analogous to a transformer in which thesecondary impedance reflected into the primary is inversely proportionalto the value of the secondary impedance. Therefore, the high secondaryresistance represented by the low conductivity metal reflects as a lowresistance into the coil to increase the circuit's time constant.

[0067] Another important characteristic of the decay oscillograms ofFIG. 8 is that the oscillogram I for iron and other ferrous materialshas a decay rate which is substantially less than the decay rate N forno target. The reason ferrous metals extend the decay time is because oftheir high permeability thus increasing the inductance of the coupledcircuit. The increased inductance means more energy stored and thereforea longer time delay for dissipating that energy.

[0068] Yet another important characteristic of the decay oscillograms isthat the decay oscillogram S for the high conductivity metals, such assilver, pure gold and copper, exhibits a substantially more rapid rateof decay than the decay rate for oscillogram N for no target. This maybe explained as the reflection of the low resistance of the highconductivity target into the coil as a high resistance, thus reducingthe time constant L/R. It may also be explained by observing that eddycurrents reflect a back emf into the coil, which opposes coil current.This mutual coupling reduces the apparent inductance of the coil.Because the eddy currents in a high conductivity metal take longer todissipate (because of the lower resistance) the back emf induced in thecoil by the eddy currents is greater for a longer time interval than formetals of lesser conductivity. High conductivity metals have an effectopposite the effect of ferrous metals because they reduce the apparentinductance of the coil mutually coupled with the metal object more thanmetals of less conductivity. Metals of intermediate conductivity, suchas 14 karat gold, an alloy commonly used in jewelry and plotted in FIG.8 as oscillogram G, or aluminum alloy decay at an intermediate ratebetween the decay rates for the high conductivity metals and the ferrousmetals.

[0069] Consequently, as the inductor 52 is moved from a position remotefrom a buried metal object, at which there is essentially no coupling tothe object, toward the buried metal object, samples taken in this earlyinterval, for example at time T1, will increase in magnitude if themetal is ferrous, decrease in magnitude if the metal is one of highconductivity, such as pure gold or silver, and not change much in thepresence of a jewelry gold metal object. Although the intermediateconductivity metal object sample magnitude will not change much as theobject is approached by the metal detector, the samples at time T4,which indicate range, will increase as that object is approached,thereby signaling that a metal object is being approached.

[0070] The signaling circuit 58 may be provided with a signalrepresenting the magnitude of samples taken at time T1 and signalschanges in that sampling magnitude. Consequently, the user will be awarethat a metal object is present by a variation in the signal from therange signaling circuit 60 and, knowing that, the user will be able todetermine the nature of that metal object by the changes, or lack ofchanges, in the signal from the metal type signaling circuit 58.

[0071] The time interval in which the sampling is done to detect thetype of metal in the buried object may be seen with reference to FIG. 8.After several time constants, the decaying oscillograms for all metalsare greater than the decaying oscillogram for no target N. The decayoscillograms for high conductivity and medium conductivity metalsinitially follow a path to the left of the oscillogram N and eventuallyintersect it. The decay oscillogram for ferrous metals remains to theright of the oscillogram N. It is therefore necessary that samples takenfor the purpose of detecting the type of metal must be taken before thehigh conductivity metal oscillograms intersect the no target oscillogramN at time T5. Therefore, the sampling time for obtaining a sample todetect the nature of the metal must occur within a time intervalbeginning after termination of the pulse at time T0. That time intervalends before the time at which the inductor current in the presence of ahigh conductivity metal object decays to a current which exceeds thecurrent to which the inductor current decays in the absence of a metaltarget object; i.e. before time T5 in FIG. 8.

[0072] An ideal time position within that time interval to sample thecoil decay voltage would appear to be time T1, which, in the embodimenttested, is approximately 7 microseconds after T0. The time T1 ispreferred because it is the time of intersection between the no targetoscillogram N and the oscillogram G for 14K gold. By utilizing thesampling time T1, samples for 14K gold would not vary in magnitude fromsamples taken for a no target condition and this lack of change wouldsignal to the user that the target being detected by the range sample attime T4 may be a 14K gold target. The exact time T1 at which the goldoscillogram G and the no target oscillogram N intersect is a function ofcircuit values, including the resistive energy damper 54 and theinductance of the inductor 52. Therefore, different sampling times willbe used with different specific circuits embodying the presentinvention.

[0073] Although the circuit operates well by taking a single sample attime T1 within the time interval between T0 and T5, experimental usageof such an embodiment of the invention has indicated that it issometimes difficult to precisely locate the sample time T1 at the exactintersection of oscillograms G and N. That fact, plus other circuit andenvironmental condition variables, sometimes makes it difficult todetermine whether the magnitude of the sample taken at time T1 isincreasing to indicate the presence of an iron target, decreasing toindicate the presence of a high conductivity target, or is changing onlyin a small amount as a result of the circuit variables to indicate thepresence of an intermediate conductivity metal, such as 14K gold.

[0074] As a consequence, the sampling method may be further improved bytaking two samples at times T2 and T3 in the time interval between T0and T5. The time position for this sampling at time T2 and T3 isslightly before time T1 and slightly after time T1 and in the presentinvention is preferably at 5.85 microseconds following T0 and 8.3microseconds following time T0. These two sample times are alternatelyselected by the user by switching a manual switch between two positions.

[0075] Consequently, in operating the circuitry, the user will first usean embodiment of the invention with the metal-type selecting sample timeat either T2 or T3. Upon perceiving a signal indicating a variation insample magnitude for samples taken within the interval T0-T5, forexample at T2, the user mentally notes the direction of change. The userthen switches to the other sample time, for example T3, and compares thesignaled direction of change to the previously noted direction. If thedirections of change of the sample magnitude signaled to the user is thesame in both instances, for example an increase in both instances, theuser will know that a ferrous metal has been detected. Conversely, if adecrease in sample magnitude is signaled to the user for both samplestaken at time T2 and time T3, the user will know that a highconductivity metal, such as silver, is being detected. However, if thedirection of change is different for one sample time, for example T2,than it is for the other sample time, for example T3, the user will knowthat a metal of intermediate conductivity has been detected.

[0076] From the above discussion of the theory of operation of theinvention, it can be seen that the invention presents a manner ofmeasuring the effect of a change in the apparent inductance of the coilinductor, resulting from mutual coupling with the metal object. Thecircuit detects a signal, which is a function of the coil's apparentinductance resulting from mutual coupling between the coil and the eddycurrents circulating in the metal object. The circuit then signals thedirection and magnitude of change in the apparent inductance of theinductor as the probe is moved. This is done by monitoring a signal,such as coil voltage, which changes as a function of a change inapparent coil inductance.

[0077] The change in apparent coil inductance can be used to signal thetype of metal for the following reasons. For buried metal objects of anyconductive material, eddy currents are induced in the object when theeddy currents and the coil are mutually coupled and the coil is excitedby a time changing current. The mutual coupling reduces the apparentinductance of the coil and does so as a function of the conductivity ofthe conductive material in the buried object. The higher theconductivity of the buried object, the more the apparent inductance ofthe coil is reduced.

[0078] With ferromagnetic materials although the same phenomenon ispresent, there is simultaneously also an additional phenomenon that theinductance is increased because of the increase in permeabilityresulting from the high permeability of ferromagnetic materials. Becausethe conductivity of ferromagnetic materials is relatively low, theeffect of an increased inductance caused by the permeability offerromagnetic materials is considerably greater than the effect of areduced inductance caused by the mutual coupling with a conductivematerial. Therefore, ferromagnetic materials exhibit a net increase inapparent inductance of the coil. However, in the absence offerromagnetic permeability, buried metal objects cause a decrease inapparent inductance and the magnitude of that decrease is indicative ofthe conductivity of the metal in the buried metal object. Consequently,the direction of change in apparent inductance of the coil together withthe magnitude of that change can be used to signal the type of metal.

[0079] The inductors used in most prior art metal detection equipmentare coils. The present invention includes the use of high permeabilitycores for increasing the inductance of the inductor. The inductorpreferred in the present invention is a coil with a ferrite core, suchas commonly used as an unshielded RF choke on printed circuit boards.The axis of the inductor is preferably parallel to the axis of theprobe. A distinct and advantageous feature of the present invention isto provide a radially asymmetric magnetic field pattern about theinductor and therefore about the axis of the probe in order to havedirectional sensitivity, which can be used to detect the bearing to theburied object.

[0080]FIG. 4 illustrates a coil 70 with a ferrite core 71 of the typedescribed above, but modified by beveling its ends 72 to provide anasymmetric shape to the core. This asymmetry slightly reduces themagnetic field in one radial direction with respect to a magnetic fieldmaximum in the diametrically opposite direction. The use as the searchcoil of a small, commercially available inductor of the type commonlysold for use as an RF choke or in resonant circuits provides a coilslightly greater than ⅛″ in diameter and ¾″ long, thus allowing theprobe diameter to be small enough for easy insertion into the soil.Although this size coil is preferred, a larger coil may be used, but thecoil is preferably no larger than ¼″ in diameter so that the probediameter will not exceed {fraction (3/81)}″ in diameter, as previouslyexplained.

[0081] An alternative manner of attaining radial asymmetry of themagnetic field is illustrated in FIG. 5, in which a coil 74 is radiallyoffset from the center of the probe 76. Each of the inductors or coilsmay be held in place within the chamber with a suitable epoxy.

[0082]FIG. 6 illustrates yet another alternative manner of accomplishingthe radial asymmetry. In that embodiment, the coil 78 is centrallypositioned within the probe 80 but a ferromagnetic material, such as aniron bar 82 is also positioned in the probe 80 parallel to its axis. Theiron bar 82 provides a low reluctance flux path so that the magneticfield radially outward from the iron bar 82 is reduced. The result is afield pattern which is asymmetric about the central axis of the probehaving a major lobe 84 of maximum sensitivity extending radiallyoutwardly from the side of the probe 80 opposite the iron bar 82.

[0083] The radial asymmetry of the magnetic field about the axis of theprobe permits rotation of the entire hand tool about the axis of theprobe, after the probe has been inserted into the soil, in order tocause a resulting variation in the detected signals, such as the samplemagnitudes. When a buried metal object is detected, the probe is rotateduntil the maximum signal is received, the maximum signal correspondingto the apparent closest range to the object. The bearing from theinserted probe to the buried object is now aligned with the direction ofthe major lobe. That direction is marked by suitable indicia on thetool, as described above. The angular direction from the probe to thetarget is the bearing.

[0084] The asymmetric field pattern feature of the present invention,referred to above and described in more detail below, for providingdirectional sensitivity can be applied to prior art metal detectioncircuitry, as well as to circuitry of the present invention.

[0085] As stated above, an alternative manner of delaying or extendingthe decay time of the coil current is to utilize, as the resistivedamper, a nonferrous, high resistivity material, in close proximity to adetecting coil. “Nonferrous” means a metal having a very lowpermeability. For the present invention, the permeability should beunder 1.5. Such a material, for example stainless steel or titanium, hasthe additional advantage that it makes a probe which is stronger andharder and therefore more durable and long lasting as it is forced intohard grounds under substantial bending forces and undergoes repetitiveabrasion in soil materials. Stainless steel #305 and grade 5 titaniumare most preferred.

[0086] Because such a metal is nonferromagnetic, it does not shield themagnetic field from the coil. A nonferromagnetic probe material storesenergy in the magnetic field generated by eddy currents induced in itand has a high resistivity permitting it to attenuate that energy, in amanner which extends the decay time of the oscillograms of FIG. 8 in thesame manner as accomplished by a shunt resistance. If a metal used toconstruct the probe were of medium or high conductivity (lowresistivity), it would extend the decay times (reduce the decay rate) bytoo much. Such a metal would extend the decay times beyond the time atwhich the eddy currents induced in the buried metal object decay to nearzero, thus preventing detection of the range of the buried metal object.

[0087] The resistivity of a metal used as a probe should be 40microohms-cm or higher and preferably more than 70 microohms-cm. Forexample, grade 5-titanium alloy has a resistivity of 177 microohms-cmand works well and is stronger than stainless. Grade 9 has a resistivityof about 140 microohms-cm.

[0088]FIG. 9 is a block diagram illustrating the preferred embodiment ofthe invention. The pulse generator 89, enclosed by dashed lines,comprises an astable, multivibrator 90, operating at 240 Hz, the outputof which is applied to a divide by two flip-flop 92 to generate a 120 Hzsignal, which in turn is amplified by the amplifier 94 and applied tothe inductor 96. The resistive damper can be either a shunt resistance97 or a high resistance, nonferromagnetic probe material as describedabove. The circuit also has a sampling circuit 98, enclosed by dashedlines, and a signaling circuit 100, also enclosed by dashed lines.

[0089] The sampling circuit 98 includes a first timing circuit 102, inthe form of a monostable multivibrator, having its input connected toreceive 120 Hz pulses from the flip-flop 92 of the pulse generator 89.The sampling circuit also has a second timing circuit 104, also in theform of a monostable multivibrator, having its input connected to the240 Hz output of the astable, multivibrator 90 of the pulse generator89. The first timing circuit 102 is connected to control a switch 106for sampling the coil voltage within the above-described time intervalfor the purpose of detecting the type of metal in a buried metal targetobject. These samples are stored on sample storage capacitor CS2.

[0090] Similarly, the second timing circuit 104 is connected to controla sampling switch 108, for sampling the coil voltage well after theabove-described time interval for detecting samples from which thepresence of the buried metal object and its range can be detected.

[0091] The voltage across inductor 96 is amplified by a low-levelamplifier 110, sampled by switch 108 and the sample is applied throughDC blocking capacitor CB to an absolute value amplifier which is made upof a high gain amplifier 112 followed by op-amp 114.

[0092] As a result, a DC signal proportional to the range sample levelsampled by switch 108 is applied through amplifiers 112 and 114 to avoltage controlled oscillator 116. The voltage controlled oscillator 116is designed to generate an audio frequency at approximately 200 Hz forthe lowest anticipated range sample magnitude and 3500 Hz for thehighest anticipated range sample amplitude, with interposed range sampleamplitudes generating correspondingly interposed frequencies. Obviouslya different frequency range may easily be utilized.

[0093] The metal type samples stored on capacitor CS2 are appliedthrough amplifier 118 to the input of a second voltage controlledoscillator 120. Amplifiers 114 and 118 have conventional thresholdcontrols 122 and 124 respectively for setting the sample level belowwhich the output of the amplifiers 114 and 118 remain at their lowestlevels so that the voltage controlled oscillators 116 and 120 do notrespond to them. Only sample magnitudes greater than the adjustablyselected threshold values will cause an increase in the output frequencyof the voltage controlled oscillators 116 and 120. This performs asquelch operation which eliminates response of the signaling circuit tonoise or other meaningless low power signals.

[0094] The audio output from the voltage controlled oscillator 116 isapplied through a switch 126 to an audio amplifier 128 and a speaker130. However, the switch 126 has its control input connected to theoutput of the voltage controlled oscillator 120, so that it isperiodically switched to alternately connect and disconnect the audiotone from the speaker at the frequency of the voltage control oscillator120.

[0095] In the operation of the circuit of FIG. 9, the astable,multivibrator 90 applies 240 Hz pulses to the monostable, multivibrator104 at 240 Hz and, through the divide by 2 flip-flop 92, applies 120 Hzpulses to the monostable, multivibrator timing circuit 102, as well asthrough the amplifier 94 to the inductor 96. The monostable,multivibrator timing circuit 102 applies a pulse of 0.5 microsecondspulse width to the switch 106, approximately 7 microseconds aftertermination of the 120 Hz pulse. This closes the switch 106 for 0.5microseconds, applying a voltage sample to capacitor CS2, which is heldon the capacitor CS2 for determining metal type when the switch 106 isopened. Similarly, the multivibrator timing circuit 104 applies a pulseof 5 microsecond duration to the switch 108 at approximately 34.1microseconds after the termination of the pulse applied to the inductor96. This causes a sample voltage to be applied through capacitor CB tothe amplifier 112 for determining range.

[0096] The range samples require significant amplification by theamplifier 110 because they are taken after nearly all decay of the coilvoltage has occurred. Because the amplifier 110 is a very high gainamplifier, having a gain on the order of 1,000, drift resulting fromaging and temperature changes in the associated electronic componentscan cause a significant error in the sampling signals. Consequently, theswitch 108 is switched at a 240 Hz rate, with the result that it isclosed, not only during the desired sampling time at 34.1 microsecondsfollowing termination of the pulse, but also at a time before the next120 Hz pulse is applied the inductor 96 and after the voltage oninductor 96 has decayed to essentially zero. This allows a DC levelcorresponding to the drift to accumulate on the capacitor CS1 so thatonly the magnitude of the range sample itself is coupled through theblocking capacitor CB to the amplifier 112. Those range samples areapplied to the absolute value amplifier, comprising amplifiers 112 and114, as a rectangular wave centered on zero volts. This rectangular wavehas an amplitude representing the range sample magnitude and isconverted to a DC level proportional to that range sample magnitude bythe absolute value amplifier.

[0097] As a result, the output 132 of the sampling circuit provides a DClevel corresponding to the magnitude of samples taken within theabove-described time interval, the magnitude of which represents thetype of metal in the buried metal object. Similarly, the output 134 ofthe sampling circuit 90 has a DC level corresponding to the magnitude ofsamples taken after the above time interval and represents the presenceand range of a buried metal object.

[0098] Since the range samples at terminal 134 are applied to controlthe frequency of voltage control oscillator 116, the tone of thefrequency emitted from the speaker 130 signals the presence and range ofthe buried metal object in a manner corresponding to the signal emittedfrom conventional metal detectors. In particular, the frequency of thetone increases as the sample magnitude increases to signal that theinductor 96 is getting closer to a buried metal object.

[0099] However, this audio tone is switched on and off at a rate whichis a function of the magnitude of the metal type samples provided atoutput 132 of the sampling circuit 98. This very low switching rate,preferably in the range of 2-12 Hz, periodically interrupts the tone tosend bursts of audio tone to the speaker 130. The user can hear thatinterruption rate.

[0100] Therefore, the user, before utilizing the embodiment of theinvention, first sets the threshold circuit 124 to an intermediate ratebetween 2 and 12 Hz. As a metal object is approached, which is signaledby a change in the audible frequency from voltage controlled oscillator116, the user can tell whether the low frequency switching rate fromvoltage controlled oscillator 120 increases, decreases or remains aboutthe same. If the switching rate of voltage controlled oscillator 120alternately switches the audio tone on and off at a reduced switchingrate, the user can recognize that a highly conductive metal, such aspure gold, silver or copper, is being approached because the samplemagnitude will be decreasing. Similarly, if the switching rate of thevoltage controlled oscillator 120 increases, the user can perceive thatthe metal object being approached is a ferrous metal because the samplemagnitude will be increasing.

[0101] If the alternate switching rate of the audio tone changes verylittle or not at all, the user will know that the metal being approachedis one of intermediate conductivity, such as aluminum alloy or 14K gold.

[0102] For providing two alternatively user selectable metal typesampling times T2 and T3, as described above, monostable multivibrator102 may simply have two, alternatively selectable timing resistancesconnected so either timing resistance can be switched into the circuit.This may, for example, be accomplished by using two series resistors,one of which is shunted by a manually operable switch 135, in FIG. 9, sothat the series resistors exhibit one resistance when the switch isopened and a lesser resistance when the switch is closed. This is alsoshown in the detailed circuit of the preferred embodiment illustrated inFIGS. 10-13.

[0103] In the event a designer prefers to use a pulse induction systemusing two coils, the connection between the output of amplifier 94 andinductor 96 is omitted and instead a transmitting coil 96A is connectedto the output of amplifier 94, as shown in phantom in FIG. 9. In thatcase, the inductor 96 becomes the receiving coil.

[0104] While the switches are illustrated by conventional schematicsymbols, the term “switches” is used in its usual broad, electronicmeaning and is not limited to mechanical switches. In fact, transistorswitches or gates are preferred. Of course, as known to those skilled inthe art, the switching function can be performed by computer software toaccomplish the equivalent result. Similarly, as is known to thoseskilled in the art, the switching and sampling functions, audio signalgeneration, sample magnitude comparison, frequency variation andintermittent burst switching, as well as the pulse generator andsampling timing circuits, can all be equivalently performed using asuitably programmed, digital computer or microprocessor.

[0105] Turning now to the operation of an embodiment of the invention, aperson using the present invention first uses a conventional metaldetector to find a buried metal target object and to determine itslocation as precisely as is possible. An embodiment of the invention isthen grasped around the handgrip and the soil is pierced as the probe isinserted down into the soil at that location. If the probe strikes ahard object and the object is metal, variations in the signals,described above, will occur as the coil located in the chamber of theprobe approaches the buried object and will signal that the object is ametal target and the type of metal, as described above. If the objectstruck is a stone or tree root, the signals will not vary appreciably.

[0106] However, if the probe does not strike a hard object in the soil,the user then alternately raises and lowers the probe within the soiland, if the metal object is near the probe, variations in the audiofrequency of the range signal can be noted. The user raises and lowersthe probe seeking the maximum frequency and can then read thegraduations to determine the depth below the ground surface of theburied metal object.

[0107] After determining the depth of the buried metal object, the userthen rotates the tool of the present invention about the central axis ofthe probe to obtain a maximum audio frequency. The indicia on the probeindicating the major lobe of sensitivity then will indicate thedirection the user should move the probe to again insert it into thesoil. The user can use conventional geometrical locating techniques,such as those used with radio direction finders, to plot or mark a lineof position for the first bearing. The probe may then be inserted asecond time and the process repeated to detect a second line ofposition, making it likely that the buried metal object is located inthe vicinity of the intersection of the two lines of position.

[0108] Additionally, the type of metal object can be detected, by notingwhether the rate of the audio bursts is changing and, if so, whether itis increasing or decreasing.

[0109] Therefore, it can be seen that embodiments of the inventionreduce and minimize both the number of times the ground must be piercedby the probe, as well minimizing the number of soil excavations toretrieve a buried metal object. The volume of soil which must beexcavated is also minimized since the tool of the present inventionprovides range and bearing information with each insertion into thesoil. Because the invention allows a user to determine the type of metalbefore the soil is disturbed, the user can exercise the option to notretrieve the object, thus reducing effort and soil disturbance. Theinvention enlarges the effective detection area sensed by the probe.Instead of relying solely upon physical contact which the operator canphysically feel, as with conventional, inactive probes, the presentinvention provides range and bearing information over a larger searcharea. Furthermore, the present invention allows the discrimination ofthree types of metals, ferromagnetic metals, nonferrous, highconductivity metals and nonferrous, mid-range conductivity metals. Theseprinciples for detecting the type of metal can be applied toconventional metal detectors which detect solely above ground. Thepreferred embodiment of the invention ultimately provides an audiblesignal which is a series of bursts of an audio tone. The pitch of theaudio tone indicates the presence, range and bearing of a buried metalobject, while the pulse rate of the bursts indicates the metal type.

[0110] FIGS. 10-13 illustrate the details of the preferred embodiment ofthe invention. These details are not necessary to understand theinvention, but are included to illustrate the presently preferred modefor practicing the invention. Many of the functional groups ofelectronic components have been framed in boxes to facilitateexplanation of the operation of the circuitry. The circuit includes manyadditional features which do not form a part of the invention, but areincluded in the preferred embodiment of the invention to enhance itsoperation.

[0111] FIGS. 10-13 illustrate a single circuit with the figures beingconnected together at the connector symbols, indicated at the peripheryof the circuit. Consequently, a description of the preferred embodimentof FIGS. 10-13 will be given with simultaneous reference to all of thesefigures. The corners of the assembled schematic have dual letters, suchas UL to signify upper left, to assist in their arrangement.

[0112] The circuit is powered by battery BAT1 or a spare battery BAT2,which is alternatively selectable by double-pull, double-throw switchSW2. The power is switched on and off by switch SW3C. Block 201 providesa low battery alarm, so that when the negative power supply falls underits regulated voltage the LED D1 mounted on a control panel is turned onto signal a low battery.

[0113] Block 202 is the monostable, multivibrator timing circuit forcontrolling the sample timing and sample width for obtaining themetal-type identification sample. It corresponds to timing circuit 102of FIG. 9.

[0114] Switch SW1, in conjunction with monostable, multivibrator timingresistors R14 and R15, permit the user to alternatively select the twometal-type identification sample times described above to assist theuser in identifying the presence of middle conductivity metals.

[0115] In block 203, potentiometer R11 is the range, threshold pitchcontrol and potentiometer R9 is the metal-type identification pulse rateor burst rate threshold control. The other components in block 203adjust the sensitivity and tracking for these controls.

[0116] Block 204 has an integrated circuit regulator U5 for regulatingthe positive power supply voltage to 5 volts.

[0117] In block 205, the metal-type identification sample signal isamplified and applied to a voltage controlled integrated circuitoscillator U3 to generate the signal for switching the audio range tonein bursts in the manner which signals the metal type, as describedabove. Amplifier U4 b sets the minimum burst rate (pulse rate) at whichthe voltage controlled oscillator can oscillate to about 2 Hz.Otherwise, a slower rate would switch the audio on and off for periodslonger than two seconds, making it difficult for the user to perceivevariations in the pulse rate or giving the appearance that the sound hasstopped. The output of amplifier U4 b feeds an inhibit input on thevoltage controlled oscillator U3, holding it off until an acceptablerate signal is achieved. The switch SW4 e in series with resistor R66connected to amplifier U4 c provides the user with an option to select areduced sensitivity. Consequently, the switch SW4 e is mounted to beaccessible on the control panel of an embodiment of the presentinvention. This switch operates in conjunction with the switch in block214, described below.

[0118] Block 206 is a protection circuit which protects the circuit frombackwardly installed batteries. It turns on to provide a very lowresistance MOSFET 220 pathway only when its gate is positive.

[0119] Block 207 is the range signal sample timer corresponding totiming circuit 104 of FIG. 9. Circuits U8 b and U8 c set the sampletiming for the range sample after the termination of the current pulse;that is, at time T4 in FIG. 8.

[0120] Block 208 has a voltage-controlled oscillator U7, which iscontrolled by the magnitude of the range sample. The audio output tonefrom the voltage controlled oscillator U7 is applied to an audioamplifier U9 and from it to a speaker SPKR1. The inhibit input to thevoltage controlled oscillator U7 is driven on and off by the output ofthe metal-type identification voltage control oscillator U3, so that theswitching of the audio tone to provide the bursts is accomplished bytransistor switching circuitry within the voltage controlled oscillatorU7.

[0121] Block 209 has a switching regulator U11 with a voltage senseinput on its pin 9 (FB). Diode D2 and inductor L2 step up a three voltbattery to plus 7 volts, while diodes D2 and D11 take a switcher pulsethrough capacitor C30 and generate a minus 7 volts across capacitor C31.Op-amp U1 b senses the negative supply and provides a regulating inputto the switcher circuit U11.

[0122] Block 210 is the astable multivibrator operating to generate thepulses for pulsing the coil and corresponds to astable multivibrator 90in FIG. 9. The integrated circuit U13 forming the astable multivibratorgenerates pulses at 240 Hz, as described above. Each pulse therefore hasa duration of 160 microseconds.

[0123] Block 211 includes integrated circuit U12 and corresponds to thedivide-by-two, flip-flop 92 described in connection with FIG. 9. Theoutput of the divide-by-two circuit U12 is fed to a clamping MOSFET Q3.The MOSFET Q3 holds the gate of the output MOSFET Q2 at ground potentialevery other timing cycle. This provides an operating cycle thatalternates between a sample-representing eddy current from a target anda sample representing no eddy current. This difference is used in block216 to eliminate the effects of DC drift in the low-level amplifier U14,as described above. Block 212 provides a resistor network which adds asmall component of the square wave output of integrated circuit U12 tothe low level input to integrated circuit U14. This bias assures thatthe approach to a target always results in an increasing signal, insteadof a slight decrease followed by a large increase. This is because theresidual, no-eddy-current signal can be a slight residual over thetarget-signal that the absolute value detector (U4 b and U4 a in block216) will see in error as a positive target signal.

[0124] In block 213, the inductor L1 is the search inductor andcorresponds to the inductor 96 of FIG. 9. It is the main coil. ResistorR65 and potentiometer R41 provide the resistive damper, described above,in a variable or adjustable manner. Diodes D5 and D6 provide the voltageclamp for the high voltage of the coil spike for protecting the circuitelements, as described above.

[0125] Block 214 is a low-level amplifier using op-amp U14. It isnecessary that it have an extremely fast response time in order torecover from the overload caused by the voltage spikes which appear onthe coil. Switch SW4 d provides a lower gain setting and operates inconjunction with switch SW4 a in block 205.

[0126] Integrated circuit U10 is the high speed-sampling switchdescribed above, although only two of its four switching sections areused.

[0127] Block 216 uses integrated circuit U6 a to buffer the range samplestored on range sampling capacitor C32. The range signal is a squarewave at about 120 Hz as a result of the on/off action of thedivide-by-two circuit in block 211. The difference between the high andlow square wave levels is a function of the signal strength from a metaltarget. The DC component is still present on pin 1 of integrated circuitU6 a, but it is removed after passing through capacitor C29. Integratedcircuits U4 b and U4 a detect the absolute value of the range signal,while using diode D7 and D8 to provide a nonlinear gain adjustment tocompensate for a signal that increases by a factor of 64 whenever thedistance to the target is halved; that is, the signal is inverselyproportional to the target distance raised to the power of 6.

[0128] Block 217 provides a regulated minus 5 volts from the minus 7volt power supply.

[0129]FIG. 11 also illustrates the circuitry for connecting a headset toa metal detection circuit of the present invention or alternatively to aconventional second metal detector, so that the user can use both theconventional metal detector and the embodiment of the invention withoutchanging headsets.

[0130] A double-throw switch SW3 a,b, which is preferably a double-poleswitch for maintaining left and right channels for the above groundconventional metal detector, has input terminals 301 and 303 connectedto a connector 305 for connection to the audio output of a second,conventional metal detector for connecting the audio from theconventional metal detector to a headset connector 307 when the switchSW3 a,b is in the position illustrated in FIG. 11. Alternatively, whenthe switch SW3 a,b is switched to its other position, the headsetconnector 307 is connected to the audio output from amplifier U9 in theblock 208. Switch SW3 a,b operates simultaneously with switch SW3 c,illustrated in FIG. 10, to switch off battery power to the embodiment ofthe invention when the headset is switched into connection with aconventional metal detector.

[0131] Temperature Compensation

[0132] There is a need to stabilize the metal type identification signalas the tip temperature changes so that the metal identification signaldoes not change as a result of temperature change. If there is notemperature compensation, the metal type identification signal willchange when the probe is inserted into a soil which is at a differenttemperature than the probe, which has previously reached thermalequilibrium with the surface air. The metal type identification signalchanges with temperature, in the absence of thermal compensation,principally because of coil resistance changes resulting in a change inthe energy stored in the magnetic fields associated with the probe. Inaddition to the change in stored inductive energy resulting from thechange with temperature of the coil's resistance, there is atemperature-dependant change in the ferrite core's permeability whichalso results in a change in stored energy. A change in stored energycauses a change in the detected signals, such as a sample magnitude,which in turn causes misleading or confusing changes to the signalsoutput to the user.

[0133] More particularly, in the uncompensated metal detector describedabove, the metal type identification feature is greatly affected by thetemperature of the sensing inductor. A small (1 deg. C.) change at theprobe tip causes a noticeable change in the audio pulse rate that isused to signal the detected metal type to the user. Since the ambienttemperature in the ground is usually different from air temperature, theuser must wait for the tip temperature to stabilize after inserting itinto the ground. He must then adjust the pulse control to give a slowpulse rate so that he can observe the change that occurs when heapproaches an underground target.

[0134] The target's metal type is determined by measuring the decayingvoltage that develops across the probe's inductor at a specific timeafter a current through the inductor is turned off as described above. Anearby target influences the energy stored in the probe's inductor byincreasing or decreasing the permeability of the entire flux path andtherefore the inductance of the inductor. However, the stored energy andthis measured voltage is proportionally influenced by the currentflowing through the inductor during the 165 microsecond pulse whichprecedes the decay illustrated in FIG. 8. In the above describedcircuit, this current is proportional to the resistance of the probe'sinductor. This resistance changes with temperature from about 10 ohms atroom temperature to about 9.5 ohms at 0 deg. C.

[0135] The preferred circuit described below and shown in the drawingsappended as FIGS. 15 and 16 compensates for all temperature effects. Aconstant current circuit shown in FIG. 17 compensates for the change inthe resistance, but does not compensate for the shift in the ferritecore's permeability, and some temperature effect is still noticeable.

[0136]FIGS. 15 and 16 include a full schematic and a simplified diagramthat illustrate this temperature compensating improvement to theinvention. The improved circuitry shown in FIGS. 15 and 16 measures andprecisely balances any change in the metal type identification signalcaused by the change in the probe inductor's resistance and theferrite's permeability due to temperature changes.

[0137] Referring to FIG. 15, a 1 ohm resistor R68 is connected in serieswith the probe's inductor L1. Approximately 1 amp current flows throughthis inductor during the 165 microsecond pulse applied for the pulseinduction detection method. This current develops a voltage across R68proportional to the current through L1 and, because the resistance ofR68 is much less than the resistance of L1, proportional to L1'sresistance. This voltage is sampled at the end of the 165 microsecondpulse, after the current has reached a steady state in the inductor L1.The sampling is performed by a section of switch U10 and the sample isheld in capacitor C41. This voltage sample is buffered by Dev3 b.Potentiometer R72 is set to balance the temperature effect.

[0138] The theory of operation of the circuit may begin with theobservation that changes in the temperature of the resistive elements inthe probe result in changes in their resistance, and therefore result inchanges in the current through those elements. By measuring the currentthrough the coil in the probe, we are effectively measuring thetemperature with changes in that current being proportional to changesin temperature. Consequently, the sampled voltage across the resistorR68 represents probe temperature and changes in that sampled voltage area linear function of temperature changes. These samples may be referredto as temperature samples.

[0139] The ground piercing metal detector is used in soils having atemperature typically in the range of 32° F. to 100° F. In thattemperature range, the thermal errors introduced into the magnitude ofthe samples taken for purpose of metal type identification areessentially a linear function of temperature over the typicallyencountered range of metal type identification sample magnitudes. As aresult, the temperature samples may be linearly scaled down to a reducedmagnitude essentially equal to the the magnitude of the thermal error.

[0140] A decrease in temperature of the probe decreases the resistanceof the coil but not the resistance of resistor R68 which is in thecircuitry located in the detection circuit 28 (FIG. 1) located aboveground. The coil resistance decrease causes an increase in the currentthrough both the coil and the resistor R68. This current increaseresults in an increase in the thermal sample magnitude and an increasein the energy in the coil and the energy coupled into any metal objects.The increase in the total energy causes an increase in the magnitude ofthe sample taken for detecting the metal type as described above. Thenet effect is that both the thermal sample magnitude and the metal typedetection sample magnitude increase with a temperature decrease. Theopposite is true for temperature increases. Therefore, since all thesechanges are essentially linear, a scaled down proportion of the thermalsample may be subtracted from the metal type detection sample to correctthe error introduced by temperature changes. This subtraction isperformed by applying both the scaled down error sample and the metaltype detection sample to the input terminal 15 of the differentialamplifier U4 c in FIG. 16. The input circuit to terminal 15 is connectedto form a summing circuit connected to perform the subtraction. Thescaling of the thermal sample is calibrated by adjustment of thepotentiometers R70 and R72 which, along with resistor R69, forms ascaling circuit.

[0141] In summary, the thermal sample of the steady state voltage acrossR68 is a temperature signal which is used as an offset against thetemperature induced error in the metal type detection sample to balanceout and eliminate that error. The circuit is, therefore, a currentdetector which detects the coil current, by detecting a voltageproportional to that current, and then scales the detected signal andsubtracts a proportion of it from the signal derived from the coilduring the metal detection procedure. This recognizes that the currentand current variations are proportional to temperature and thereforeprovide a temperature signal which is used to compensate for thermallycaused signal variations. It should be clear that other currentdetection circuits known to those skilled in the art may be used andtheir outputs scaled and proportionally subtracted from the detectedsignal for compensating temperature variations in the probe.

[0142] In use, the user monitors the changes in the signal to detect themetal type. As described above, the user observes the direction ofchange in the pulse rate as an indicator of metal type. For example, ifthe pulse rate increases, a ferrous metal is being observed. The pulserate slows for silver. It is the direction of change and not the amountof change which is important. The amount of actual change is typicallyvery small. Therefore, the important thing is to balance out thetemperature error before the user detects any metal because this is thebeginning point from which changes are referenced.

[0143] To accomplish the accurate compensation for temperature, thepotentiometer R72 is adjusted to scale the magnitude of the thermalsample so that there is no change in the metal type signal when theprobe temperature is changed in the absence of any coupled metal object,such as by being inserted in ground away from metal objects. This may bedone as a part of the calibration of the instument under laboratoryconditions.

[0144] The verify switch changes the sampling point of theidentification circuitry to enable the circuit to identifymid-conductivity metals in the manner described above. For example, thesampling points become T2 and T3 shown in FIG. 8. Since the Verifyswitch's “A” sample point is at a different point on the inductors decaycurve illustrated in FIG. 8 than the “B” sample point, a differenttemperature compensation gain is required for switch position “A” thanis required for position “B”. One section of the Verify switch addsresistance network R70 and R 69 when the switch is in the “A” positionto change the scaling factor and eliminates that resistance network whenin the “B” position. To adjust for thermal compensation, first,potentionmeter R72 is adjusted to eliminate any metal type signal changeresulting from temperature change when the Verify switch is open in theB position. Then potentiometer R70 is adjusted to accomplish the samecondition when the Verify switch is closed in the “A” position.

[0145] Another improved circuit is shown in FIG. 17 but is consideredless of an improvement than the above circuit. A constant currentcircuit is connected in series with the probe's inductor L1 at thevoltage side (not the ground side). This circuit compensates for changesdue to temperature changes at the tip. FIG. 17 shows the use of aconstant current regulator U1 to improve the temperature stability ofthe ground piercing metal detector's metal identification feature.

[0146] With reference to the circuit shown in FIG. 17, the strength ofthe magnetic field developed by the sensing inductor L1 is proportionalto the current flowing through L1 when all other factors are constant.If the inductor were driven by a fixed voltage source, the current wouldbe determined by the resistance of L1. This resistance changes withtemperature. Therefore, the current would change with temperature. Thisparticular temperature dependence is reduced by driving the inductor L1with a commercially available constant current regulator so that as theresistance of L1 changes with temperature, the current through L1 doesnot change. This method of temperature stabilization does not compensatefor the change with temperature of the inductor's ferrite core'spermeability. Nor does it compensate for the effects of core saturationwhich occur in the invention at the currents used.

[0147] As will be apparent to those skilled in the art, the inventionmay use microprocessor technology so that many of the circuits andprocedures may be performed by the microcontroller and its software. Forexample, sample and hold circuits may be controlled by amicrocontroller, their outputs applied to the microcontroller throughanalog to digital converters and the scaling and mathematical processingmay then be performed by the microcontroller.

[0148] While certain preferred embodiments of the present invention havebeen disclosed in detail, it is to be understood that variousmodifications may be adopted without departing from the spirit of theinvention or scope of the following claims.

1. A hand tool for detecting buried objects, the tool having a hand gripattached to a ground piercing probe which is manually insertable intosoil to displace the soil radially outwardly from the probe, the toolcomprising: (a) at least one inductor positioned in a chamber formed inthe probe and connected to a metal detection circuit; (b) a currentdetecting and scaling circuit connected to the inductor to detect asignal representing the current through the inductor; and (c) a summingcircuit connected to the output of the scaling circuit for subtractingthe current signal from a metal detection signal derived from the coil.2. A tool in accordance with claim 1 wherein the metal detection circuitis a pulse induction metal detection circuit.
 3. A tool in accordancewith claim 2 wherein a resistor is connected in series with the inductorand the current detecting and scaling circuit is a sample and holdcircuit having its input connected between the inductor and theresistor.
 4. A tool in accordance with claim 3 wherein the sample andhold circuit includes a resistor network having at least one resistorwhich is alternately switchable into and out of the circuit forproviding alternate scaling factors.
 5. A method for compensating fortemperature variation in signals generated by an inductor mounted in aprobe of a ground piercing metal detector and connected to a metaldetector circuit to apply metal detection signals to the metal detectorcircuit, the method comprising: (a) detecting a signal representing thecurrent through the inductor; and (b) subtracting a scaled portion ofthe current signal from the metal detection signal.
 6. A method inaccordance with claim 5 wherein the metal detection signal is obtainedby the pulse induction method including application of a pulse to theinductor and wherein the current through the inductor is detected bysampling and holding a voltage which is proportional to the inductorcurrent at a time interval near the end of the pulse.
 7. A metaldetection circuit for detecting the metal type of a metal target object,the circuit comprising: (a) a pulse generator; (b) a coil connected tothe pulse generator for generating a time changing magnetic field inresponse to electrical pulses applied to the coil; (c) a resistiveenergy damper coupled to the coil for attenuating the energy in themagnetic field; (d) a first sampling and storing circuit connected tothe coil for sampling a signal representing the magnitude of the coilcurrent at a sampling time within a time interval beginning aftertermination of a pulse and ending before the time at which inductorcurrent in the presence of a high conductivity metal target objectdecays to a current which exceeds the current to which the inductorcurrent decays in the absence of a metal target object; (e) atemperature compensation circuit including a second sampling and storingcircuit connected to the coil for sampling a signal representing coilcurrent at a time interval during the pulse, a scaling circuit and asumming circuit for subtracting a scaled portion of the coil currentsample from the sample obtained by the first sampling and storingcircuit; and (f) a signaling circuit connected to the first sampling andstoring circuit for signaling changes in the detected coil current atthe sampling time as the coil is moved.
 8. A method for detecting themetal type of a metal target object buried below the surface of a soil,the method comprising: (a) attempting to induce eddy currents in thetarget object by applying a pulse to an inductor spaced from the targetobject to generate a time changing magnetic field about the inductor;(b) detecting a signal representing the current through the inductornear the end of the pulse; (c) detecting a signal which is a function ofcoil apparent inductance resulting from mutual coupling with the eddycurrents; (b) compensating for temperature variation in the inductor bysubtracting a scaled portion of the current signal from the apparentinductance signal; and (c) signaling the direction of change in theapparent inductance of the inductor as the probe is moved.
 9. A handtool for detecting buried objects, the tool having a hand grip attachedto a ground piercing probe which is manually insertable into soil todisplace the soil radially outwardly from the probe, the toolcomprising: (a) at least one inductor positioned in a chamber formed inthe probe and connected to a metal detection circuit; (b) a pulsegenerator connected to the inductor for periodically applying a pulse tothe inductor, the pulse generator including a constant current regulatorfor providing a constant current pulse generator.